The present invention relates to photolithography.
As the size of devices is made smaller with each successive generation of microelectronics, the thickness of a photoresist layer used in patterning features of the devices is typically reduced. This is so for a variety of reasons, including the need to compensate for reduced control over the photolithographic depth of focus and reduced depth to which the photolithographic dose is absorbed at shorter wavelengths. However, a primary reason for the reduction of the photoresist thickness is avoidance of pattern collapse in the imaged photoresist layer. A well-known mechanism causing the collapse of photoresist patterns is loss of adhesion between the photoresist patterns and an underlying layer on which they are formed. The underlying layer is frequently a substrate or a layer of a substrate which consists essentially of a polymeric material. Such polymeric material layer is generally utilized as an antireflective coating (ARC).
Generally, the thickness of the photoresist layer is determined as a fixed multiple of the smallest size of a feature to be imaged in the photoresist layer. Stated another way, the photolithography process places a maximum value on the aspect ratio of photoresist patterns, the aspect ratio being the height of a patterned feature in a photoresist layer to the minimum width of the patterned feature. For photoresists sensitive to both i-line and deep ultra-violet wavelength (DUV) sources, the maximum aspect ratio was maintained at approximately 3.5 over several generations of shrinking feature sizes. For photoresists sensitive to 193 nm sources, the maximum aspect ratio has been maintained at approximately 3.0. However, for photoresists sensitive to 193 nm with higher effective numeric apertures and 157 nm sources, the avoidance of pattern collapse may require the maximum aspect ratio to decrease to an even lower value of about 2.0 to 2.5. Given that the size of features are shrunk even further for sub 130 nm node technologies, the reduced aspect ratio means that the height of the photoresist patterns is drastically reduced from what it was only a few years before.
As a result, in order to avoid pattern collapse, the photoresist thickness is reduced drastically, sometimes at the expense of other processing parameters. At such reduced photoresist thickness, one or the other of the following responses would be required, if a conventional approach were employed. One response would be to greatly increase the selectivity of the etch process to the patterned photoresist material, an achievement which is difficult to accomplish. Another way would be to utilize additional films, e.g. hardmask layers, for use with the patterned photoresist layer in sequentially transferring the pattern to underlying layers, a solution, while workable, which results in increased process complexity.
Therefore, it would be desirable to provide a method of avoiding collapse of photoresist patterns, which does not require the height of the patterns to be constrained to the same degree as conventionally practiced.
In addition, it would be desirable to provide a method of preparing a substrate for photolithographic patterning without requiring the selectivity of etch processes to be significantly increased and without having to utilize multiple films for sequential pattern transfer.